1. Field of the Invention
The present invention relates to a double-walled pipe structure comprising a plurality of pipes of different cross-sectional shapes combined for increased mechanical strength and reduced weight.
2. Description of the Related Art
Various pipes have been used as structural parts for automobiles, for example. In various applications, pipes of different pipe diameters and wall thicknesses are selected to meet demands for different mechanical strengths and weights. If one pipe material is used in an application where it is subject to different loads at different positions, then it is the general practice to determine a pipe diameter and a wall thickness based on the mechanical strength of a pipe portion subject to a maximum load, or to join a pipe having a larger diameter or a greater wall thickness and a pipe having a smaller diameter or a smaller wall thickness to each other by welding or the like.
Even if the mechanical strength of a pipe should be varied directionally, it is customary to determine a pipe diameter and a wall thickness based on the mechanical strength of the pipe with respect to the direction in which a maximum load is applied.
According to the conventional practice of determining a pipe diameter and a wall thickness based on either the mechanical strength of a pipe portion subject to a maximum load or the mechanical strength of the pipe with respect to the direction in which a maximum load is applied, the resultant pipe tends to be too heavy as a whole and also too strong in pipe portions that are not required to be so strong.
Furthermore, it is time-consuming and expensive to carry out the process of joining a pipe having a larger diameter or a greater wall thickness and a pipe having a smaller diameter or a smaller wall thickness to each other by welding or the like.